55th Infantry Division troops blinded by tear gas during the Battle of Estaires, April 10, 1918. The Battle of Estaires was one of the eight battles of the Lys Offensive (April 7-29, 1918) and part of the 1918 German Spring Offensive in Flanders during WWI. The German assault struck the Portuguese 2nd Division, which held a front of about 17 miles. The Portuguese division was overrun and withdrew towards Estaires. The British 55th Division, to the south of the Portuguese in a more defensible position, pulled back its northern brigade and held its ground for the rest of the battle, despite attacks from two German reserve divisions. The Germans broke through 9 miles of front and advanced up to 5 miles, the most advanced probe reaching Estaires on the Lys. There they were finally halted by British reserve divisions. Chemical weapons in WWI were primarily used to demoralize, injure, and kill entrenched defenders. The earliest military uses of chemicals were tear gas (ethyl bromoacetate) rather than fatal or disabling poisons. As bromine was scarce among the Entente allies, the active ingredient was changed to chloroacetone. None of the combatants considered the use of tear gas to be a conflict with the Hague Treaty of 1899, which prohibited the launching of projectiles containing asphyxiating or poisonous gas.